1. Field of the Invention
The invention relates to a nanocomposite thermoelectric conversion material in which nanosized particles for phonon scattering are dispersed in the matrix of a thermoelectric conversion material, and a process for producing the same.
2. Description of the Related Art
A thermoelectric conversion material is an energy material that directly converts thermal energy to electrical energy based on two basic thermoelectric effects, the Seebeck effect and the Peltier effect.
Compared with conventional electric power generating technology, a thermoelectric generating device using a thermoelectric conversion material bas many advantages. For example, it has a simple structure, has a high durability, has no movable component, is easy to micronize, is highly reliable and maintenance free, has a long life, does not generate acoustic noise, does not pollute the environment, and utilizes low temperature waste heat.
Compared with conventional compression type cooling technology, a thermoelectric cooling device using a thermoelectric conversion material has advantages. For example, it does not require fluorocarbons and therefore does not pollute the environment, is easy to miniaturize, has no movable component, and does not generate acoustic noise.
Therefore, in response to the increasing severity of energy and environmental problems over recent years in particular, the practical use of thermoelectric conversion materials is expected in a wide range of applications in fields such as aerospace, national defense buildup, observation of geological and meteorological phenomena, medical services and hygiene, and microelectronics, as well as in utilization of waste heat in petroleum processing, metallurgy, and the electric power industry.
The power factor P=S2σ and the dimensionless thermoelectric figure of merit ZT=(S2σ/κ)T, where S is the Seebeck coefficient, σ is electrical conductivity, κ is thermal conductivity, and T is absolute temperature, are used as indications for evaluating the performance of thermoelectric conversion materials. In other words, to obtain high thermoelectric properties the Seebeck coefficient S and the electrical conductivity σ need to be high, and the thermal conductivity κ needs to be low.
Phonon scattering, which are carriers of thermal conduction, is effective in decreasing thermal conductivity κ, and a composite thermoelectric conversion material in which particles for phonon scattering are dispersed in a thermoelectric conversion material matrix has been advocated as a thermoelectric conversion material.
Japanese Patent No. 4286053 discloses a technology, which segregate a Te rich phase, that includes excess concentration of Te compared to stoichiometric composition, at the grain boundaries or within the grains of a BiTe type thermoelectric semiconductor. Then the thermal conductivity is lowered due to distortion of the crystal lattice caused by the Te rich phase.
However, because the segregated Te rich phase is on a micron-order, the decrease in the thermal conductivity is very small, and the thermoelectric conversion performance is not greatly increased. In this technology, synthesis is carried out using molten metals, and further reduction in grain size cannot be achieved.
WO 2007/066820 (A1) describes a technology for dispersing nanosized phonon-scattering particles in the matrix of a thermoelectric conversion material at intervals not larger than the mean free path of phonons and not smaller than the mean free path of the carrier. This document also describes technology for reducing the slurry by mixing the thermoelectric material precursor solution into a ceramic powder dispersed pH-adjusting liquid.
Japanese Patent Application Publication No. 2008-305907 (JP-A-2008-305907) describes technology for dispersing ceramic particles as nanosized phonon-scattering particles in the matrix of a thermoelectric conversion material.
Japanese Patent Application Publication No. 2008-305919 (JP-A-2008-305919) describes technology for dispersing metal particles as nanosized phonon-scattering particles in the matrix of a thermoelectric conversion material.
By reducing the phonon-scattering particles to nanosized particles, the technologies described in WO 2007/066820 (A1), JP-A-2008-305907, and JP-A-2008-305919 can decrease the thermal conductivity and greatly increase thermoelectric conversion performance over technologies utilizing micron-sized particles.